JP5802224B2 - Magnetic recording medium - Google Patents

Description

  The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having a magnetic layer in which ε-iron oxide powder, which is a ferromagnetic powder suitable for high density recording, is highly dispersed.

  As the amount of recorded information increases, magnetic recording media are always required to have a higher recording density. Therefore, in order to achieve high-density recording, it is widely performed to increase the filling rate of the magnetic layer by using a fine particle magnetic material.

  Conventionally, a ferromagnetic metal powder has been mainly used for a magnetic layer of a magnetic recording medium for high density recording. However, in order to achieve higher density recording, there is a limit to the improvement of the ferromagnetic metal powder. This is because the ferromagnetic metal powder becomes superparamagnetic due to thermal fluctuation when the particle size is reduced, and cannot be used for a magnetic recording medium.

  On the other hand, ε-iron oxide powder, which has been used mainly as a permanent magnet, has high crystal magnetic anisotropy derived from the crystal structure and excellent thermal stability. Suitable and excellent magnetic properties can be maintained. Therefore, in recent years, it has been proposed to use ε-iron oxide powder for magnetic recording (see, for example, Patent Document 1).

JP 2008-60293 A

  One of the characteristics required for a magnetic recording medium for high density recording is that the magnetic layer has high surface smoothness. For this purpose, it is important to highly disperse the ferromagnetic powder. However, permanent magnets, which have been the main application of conventional ε-iron oxide, are not required to highly disperse ε-iron oxide. Therefore, a sufficient study has not been made on a dispersion technique for dispersing ε-iron oxide so highly that it can be applied to a magnetic recording medium for high-density recording.

  Accordingly, an object of the present invention is to provide a magnetic recording medium having a magnetic layer in which ε-iron oxide powder is highly dispersed.

  As a result of intensive studies to achieve the above object, the present inventor has found that a compound having a substituent selected from the group consisting of a hydroxyl group and a quaternary ammonium base can disperse the ε-iron oxide powder highly. The present inventors have found that the compound is useful as an agent, and have completed the present invention.

That is, the above object has been achieved by the following means.
[1] A magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a nonmagnetic support,
The ferromagnetic powder is ε-iron oxide powder,
The magnetic layer further includes a compound having a substituent selected from the group consisting of a hydroxyl group and a quaternary ammonium base.
[2] The magnetic recording medium according to [1], wherein the compound is a compound selected from the group consisting of an aromatic compound having a hydroxyl group and an aliphatic compound having a quaternary ammonium base.
[3] The magnetic recording medium according to [1] or [2], wherein the compound is an aromatic compound in which a hydroxyl group is directly substituted with an aromatic ring.
[4] The magnetic recording medium according to any one of [1] to [3], wherein the magnetic layer further includes a friction coefficient reducing component.
[5] The magnetic recording medium according to [4], wherein the friction coefficient reducing component is inorganic oxide colloidal particles.
[6] The compound is an aromatic compound in which a hydroxyl group is directly substituted on an aromatic ring,
The friction coefficient reducing component is an inorganic oxide colloid particle, and
The magnetic recording medium according to [4] or [5], wherein the magnetic layer does not contain carbon black.
[7] The magnetic recording medium according to any one of [4] to [6], wherein the friction coefficient reducing component is silica colloid particles.
[8] The magnetic recording medium according to any one of [1] to [7], wherein the compound is an aromatic compound having a hydroxyl group, and the aromatic compound includes one aromatic ring.
[9] The magnetic recording medium according to any one of [1] to [8], wherein the compound is an aromatic compound having a hydroxyl group, and the aromatic ring contained in the aromatic compound is a naphthalene ring.
[10] The magnetic recording medium according to any one of [1] to [9], wherein the compound is dihydroxynaphthalene.
[11] The compound is an aliphatic compound having a quaternary ammonium base, and the aliphatic group contained in the aliphatic compound is an alkyl group [1], [2], [4], [5], Or the magnetic recording medium as described in [7].
[12] The magnetic recording medium according to [11], wherein the aliphatic compound is cetyltrimethylammonium bromide.

  As described above, ε-iron oxide is a magnetic powder suitable for high-density recording because it has high thermal stability. According to the present invention, it is possible to provide a magnetic recording medium for high density recording having a magnetic layer in which such ε-iron oxide is highly dispersed.

The present invention relates to a magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a nonmagnetic support. In the magnetic recording medium of the present invention, the ferromagnetic powder contained in the magnetic layer is ε-iron oxide powder, and the magnetic layer further comprises a compound having a substituent selected from the group consisting of a hydroxyl group and a quaternary ammonium base. Including. According to the present invention, a magnetic recording medium having a magnetic layer in which ε-iron oxide is highly dispersed can be provided by using the compound as a magnetic layer component.
Hereinafter, the magnetic recording medium of the present invention will be described in more detail.

The magnetic layer includes a ε-iron oxide powder, a binder, and a compound having a substituent selected from the group consisting of a hydroxyl group and a quaternary ammonium base, and may optionally include various additives.

As the ε-iron oxide powder, it is preferable to use a fine particle magnetic material having a particle size of 8 to 30 nm and suitable as a magnetic material for a magnetic recording medium for high density recording. The particle size is more preferably in the range of 8 to 20 nm. Here, in the present invention, the particle size means a value measured by the following method.
Using a Hitachi transmission electron microscope H-9000, the particles are photographed at a photographing magnification of 100,000, and printed on photographic paper so as to obtain a total magnification of 500,000 times to obtain a particle photograph. The target particle is selected from the particle photograph, the outline of the particle is traced by the digitizer, and the particle size is measured by the image analysis software KS-400 manufactured by Carl Zeiss. About the powder which the particle | grains aggregated, the size of 500 particle | grains is measured, and let the average value of those particle sizes be a particle size (average particle size).

In the present invention, the size of the powder such as a magnetic material (hereinafter referred to as “powder size”) is (1) the shape of the powder is needle-like, spindle-like, or columnar (however, the height is the bottom surface). In the case of larger than the maximum major axis, etc., it is represented by the length of the major axis constituting the powder, that is, the major axis length, and (2) the shape of the powder is plate-shaped or columnar (however, the thickness or height (Smaller than the maximum major axis of the plate surface or the bottom surface), and (3) the shape of the powder is spherical, polyhedral, unspecified, etc. When the major axis constituting the body cannot be specified, it is represented by the equivalent circle diameter. The equivalent circle diameter is a value obtained by a circle projection method.
The average powder size of the powder is an arithmetic average of the above powder sizes, and is obtained by carrying out the measurement as described above for 500 primary particles. Primary particles refer to an independent powder without aggregation.

The average acicular ratio of the powder is determined by measuring the short axis length of the powder in the above measurement, that is, the short axis length, and calculating the arithmetic average of the values of (long axis length / short axis length) of each powder. Point to. Here, the minor axis length is defined by the above-mentioned powder size in the case of (1), the length of the minor axis constituting the powder, and in the case of (2), the thickness or height, respectively. In the case of (3), since there is no distinction between the major axis and the minor axis, (major axis length / minor axis length) is regarded as 1 for convenience.
When the shape of the powder is specific, for example, in the case of the definition (1) of the powder size, the average powder size is referred to as an average major axis length, and in the case of the definition (2), the average powder size Is called the average plate diameter, and the arithmetic average of (maximum major axis / thickness to height) is called the average plate ratio. In the case of definition (3), the average powder size is referred to as an average diameter (also referred to as an average particle diameter or an average particle diameter).
The shape anisotropy increases in the order of (2), (3), and (1). In the case where the easy magnetization axis is oriented in the plane, it is preferable from the viewpoint of micronization to select one that can simply increase the shape anisotropy. On the other hand, in the case of orienting the easy magnetization axis perpendicular to the surface for perpendicular recording, the viewpoint of fluid orientation such as coating should be taken into account, and therefore, (2), (1) and (3) are preferable. It will become. On the other hand, from the viewpoint of thermal stability, it is preferable to take the form (3), and it is more preferable to take a spherical shape.

As a method for preparing ε-iron oxide, a method using goethite as a starting material, a reverse micelle method, and the like are known. In the present invention, ε-iron oxide is prepared using these known methods, and is used for a magnetic layer. It can be used as a ferromagnetic powder. Alternatively, commercially available ε-iron oxide can also be used.
Below, the preparation method of (epsilon) -iron oxide by the reverse micelle method is demonstrated as an example.

Preparation of ε-iron oxide by reverse micelle method
(1) Production process of iron salt particles (hereinafter also referred to as “precursor particles”) which is a precursor of ε-iron oxide;
(2) A step of coating precursor particles with a sintering inhibitor, preferably by a sol-gel method;
(3) a heating and firing step of precursor particles coated with a sintering inhibitor;
(4) A step of removing the sintering inhibitor from the particle surface of ε-iron oxide obtained by converting the precursor particles by heating and firing,
Can be included.

  In the step (1), precursor iron salt particles can be precipitated from the micelle solution by the reverse micelle method. More specifically, a surfactant and an organic solvent that does not mix with water are added to an aqueous solution in which a water-soluble salt of iron is dissolved to form a W / O emulsion, and an alkali is added thereto to precipitate the iron salt. For example, the particle size of the precipitated iron salt can be controlled by the mixing ratio of the surfactant and water. As will be described later, by heating and firing after coating the precursor particles with a sintering inhibitor, it is possible to prevent the ε-iron oxide particles from being sintered into coarse particles. Therefore, the particle size of the finally obtained ε-iron oxide particles can be controlled mainly by the particle size of the iron salt particles precipitated here.

  Examples of the water-soluble salt include iron nitrate and chloride, and examples of the alkali include sodium hydroxide, potassium hydroxide, sodium carbonate, and aqueous ammonia. Further, ε-iron oxide can control magnetic properties by substituting a part of Fe with another element. Examples of the substitution element include Al, Ga, In, Co, Ni, Mn, Zn, and Ti. Such substitutional ε-iron oxide can also be used as a ferromagnetic powder in the magnetic layer in the present invention. In the case of obtaining substitution-type ε-iron oxide by the reverse micelle method, a compound of a substitution element (nitrate, hydroxide, etc.) may be added to the micelle solution in step (1).

  Step (2) is a step of coating the surface of the precursor particles with a sintering inhibitor before heating and firing in order to prevent the particles from sintering and becoming coarse particles in step (3). From the viewpoint of uniformly coating the precursor particles with the sintering inhibitor, it is preferable to coat the precursor particles with the sintering inhibitor by the sol-gel method.

As the sintering inhibitor, Si compound, Y compound, or the like can be used. From the viewpoint of the sintering prevention effect and the ease of removal after heating and firing, it is preferable to coat the precursor particles with Si oxide. For example, when a silane compound such as alkoxysilane is added to the solution in which the precursor particles are deposited in step (1), silica (SiO ₂ ), which is a hydrolyzate of the silane compound, is deposited on the surface of the precursor particles. it can. As the silane compound, it is more preferable to use tetraethyl orthosilicate (TEOS) capable of forming silica by a sol-gel method.

  The precursor particles coated with the sintering inhibitor can be subjected to washing before the step (3) in order to remove unreacted substances (the above silane compounds and the like) from the surface of the precursor particles. The washing can be performed using water, an organic solvent, or a mixed solvent thereof.

  The precursor particles coated with the sintering inhibitor as described above are subjected to heat-firing in the step (3) after being taken out from the solution, washed, dried, pulverized, and the like as necessary. The By pulverizing, firing can be performed uniformly and the sintering inhibitor after firing can be easily removed.

  The heating and baking in the step (3) can be performed at an ambient temperature of 500 to 1500 ° C., for example. For example, the precursor particles can be converted into ε-iron oxide by oxidation reaction or the like by heating and firing the precursor particles at the above atmospheric temperature in air.

  Usually, since the sintering inhibitor remains on the surface of the particles after firing, step (4) is performed to remove the sintering inhibitor. The removal method can be appropriately selected depending on the kind of the sintering inhibitor. For example, the aforementioned silica can be dissolved and removed by a method of immersing particles in an alkali solution such as sodium hydroxide (alkali washing) or hydrofluoric acid (HF). Since hydrofluoric acid is not easy to handle, alkaline cleaning is preferably used. Thereafter, washing with water is usually performed. Washing with water can be performed with water or a mixed solvent of water and an aqueous solvent such as methanol, ethanol, acetone, N, N-dimethylformamide, N, N-dimethylacetamide, tetrahydrofuran, or the like. In one aspect of the present invention, the dispersant can be deposited on the particle surface of the ε-iron oxide powder by adding the dispersant to the aqueous solution during or after the water washing.

  After washing with water, the ε-iron oxide powder may be taken out from the aqueous solution by a known solid-liquid separation method. The extracted ε-iron oxide powder can be optionally subjected to a drying treatment.

  Alternatively, after washing with water, the solvent may be replaced with an organic solvent by a solvent replacement process. The solvent replacement treatment can be performed by a known solvent replacement method in which addition of an organic solvent and solid-liquid separation are repeated.

  Regarding the preparation of ε-iron oxide particles by the reverse micelle method described above, the examples described later can also be referred to.

  The magnetic layer contains a binder together with the ε-iron oxide powder and the compound having the substituent. As the binder for the magnetic layer, a known binder used for the magnetic layer of the coating type magnetic recording medium can be used. Specifically, polyurethane resins, polyester resins, polyamide resins, vinyl chloride resins, acrylic resins copolymerized with styrene, acrylonitrile, methyl methacrylate, cellulose resins such as nitrocellulose, epoxy resins, phenoxy resins, A single resin or a mixture of a plurality of resins can be used from polyvinyl alkylal resins such as polyvinyl acetal and polyvinyl butyral. Among these, preferred are polyurethane resins, acrylic resins, cellulose resins, and vinyl chloride resins. These resins can also be used as a binder in the nonmagnetic layer described later. JP, 2010-24113, A paragraphs [0029]-[0031] can be referred to for the above binder. It is also possible to use a polyisocyanate curing agent together with the resin.

The magnetic recording medium of the present invention includes a compound having a substituent selected from the group consisting of a hydroxyl group and a quaternary ammonium base, together with ε-iron oxide and a binder. By containing the compound, the ε-iron oxide powder can be highly dispersed, and thereby a magnetic layer having excellent surface smoothness can be formed.
Hereinafter, the compound will be described in more detail.

The said compound is used as a magnetic layer component 1 type or in combination of 2 or more types.
The magnetic recording medium of the present invention preferably contains the compound in the magnetic layer in an amount of 1.5 parts by mass or more per 100 parts by mass of the ferromagnetic powder from the viewpoint of improving the dispersibility of the ε-iron oxide powder. Since it is desirable to increase the filling rate of the ferromagnetic powder from the viewpoint of high density recording, it is preferable to reduce the additive amount within a range where the effect can be exhibited. From this viewpoint, the compound content of the magnetic layer is preferably 10 parts by mass or less per 100 parts by mass of the ferromagnetic powder. From the viewpoint of achieving both the dispersibility and the filling rate of the ferromagnetic powder, the content of the compound in the magnetic layer is more preferably 3 to 10 parts by mass per 100 parts by mass of the ferromagnetic powder.

  The number of substituents selected from the group consisting of a hydroxyl group and a quaternary ammonium base contained in the compound may be one or more, and may be two or three or more. One or two is preferable because it exhibits an appropriate adsorption force.

  The compound may be an aliphatic compound or an aromatic compound. In addition, it is preferable that the said compound is not a high molecular compound used as a binder. This is not desirable from the viewpoint of high-density recording because the filling rate of the magnetic material decreases as the additive component used in the magnetic layer increases. This is because addition is required. In order to obtain an excellent effect of improving dispersibility with a small addition amount, when the compound is an aromatic compound, it is preferable that one aromatic ring is contained in one molecule. Here, an aromatic ring is a ring assembly in which two or more rings are connected through a single bond, and is counted as one, but for two or more rings connected through a connecting group other than a single bond, The included aromatic ring is counted as a plurality. For the same reason, the compound preferably has a molecular weight of 1000 or less, more preferably 500 or less. The lower limit of the molecular weight of the compound is not particularly limited, but the lower limit can be, for example, 100 or more, or 150 or more.

  From the viewpoint of further improving the dispersibility of the ε-iron oxide powder, the compound having a hydroxyl group is preferably an aromatic compound having a hydroxyl group. The aromatic ring contained in the aromatic compound having a hydroxyl group may be a monocyclic structure or a polycyclic structure, and may be a carbocyclic ring or a heterocyclic ring. In addition, the polycyclic structure may be a condensed ring or a ring assembly in which two or more rings are connected via a single bond. Specific examples of the aromatic ring include naphthalene ring, biphenyl ring, anthracene ring, pyrene ring, phenanthrene ring and the like, and preferable aromatic rings include naphthalene ring, biphenyl ring, anthracene ring and pyrene ring. More preferred aromatic rings include naphthalene rings and biphenyl rings.

  In the aromatic compound having a hydroxyl group, a hydroxyl group may be directly substituted on the aromatic ring described above, or may be substituted via a linking group such as an alkylene group such as a methylene group or an ethylene group. From the viewpoint of further improving dispersibility of the ε-iron oxide powder, it is preferable that the hydroxyl group is directly substituted with an aromatic ring.

  The aromatic ring may contain a substituent in addition to the hydroxyl group. Although it does not specifically limit as this substituent, For example, a halogen atom (for example, a fluorine atom, a chlorine atom, a bromine atom, an iodine atom), an alkyl group can be mentioned. However, an excessively high adsorption force of the compound added as a dispersant to the magnetic particles is not preferable because the association of the magnetic particles may be promoted. From this point of view, the presence of a substituent (for example, a sulfonic acid group or a salt thereof) having a higher adsorptivity to the magnetic particle surface than the hydroxyl group is not preferable. In addition, the presence of a substituent that greatly affects the hydrophilicity / hydrophobicity of the compound is also undesirable. From the above viewpoints, the aromatic compound preferably has no substituent other than a hydroxyl group.

  As the compound having a hydroxyl group described above, naphthalene substituted with a hydroxyl group is preferable, and dihydroxynaphthalene is more preferable.

On the other hand, the compound having a quaternary ammonium base is a compound represented by the following general formula, and R in the quaternary ammonium cation represented by —N ⁺ R ₃ is, for example, an alkyl having 1 to 5 carbon atoms. Group, preferably a linear alkyl group having 1 to 3 carbon atoms. Three Rs may be different from each other, or two or all of them may be the same.

The anion X ⁺ that forms a salt with the ammonium cation is not particularly limited, but halogen anions such as Cl ⁻ and Br ⁻ are preferable from the viewpoint of availability.

  From the viewpoint of further improving dispersibility of the ε-iron oxide powder, the compound having a quaternary ammonium base is preferably an aliphatic compound in which R ′ in the general formula is an aliphatic group. The aliphatic group represented by R ′ is preferably a linear or branched alkyl group. The aliphatic group preferably has about 10 to 20 carbon atoms. The aliphatic group may optionally contain a substituent such as a halogen atom. When the aliphatic group represented by R ′ has a substituent, the carbon number of the aliphatic group means the carbon number of the portion excluding the substituent.

  As the aliphatic compound having a quaternary ammonium base, cetyltrimethylammonium bromide (CTAB) is particularly preferred from the viewpoint of availability and improvement in dispersibility of the ε-iron oxide powder.

  The compound having a substituent selected from the group consisting of a hydroxyl group and a quaternary ammonium base described above is available as a commercial product, and can be easily synthesized by a known method.

  If necessary, an additive can be further added to the magnetic layer. Examples of the additive include an abrasive, a lubricant, a dispersant / dispersion aid, an antifungal agent, an antistatic agent, an antioxidant, and carbon black. As the additive, commercially available products can be appropriately selected and used according to desired properties.

  By the way, as a characteristic desired for the magnetic recording medium, in addition to having a high surface smoothness, the magnetic layer has excellent running durability. For this purpose, it is preferable that a component for reducing the friction coefficient of the magnetic layer (friction coefficient reducing component) is included in the magnetic layer. In the present invention, the “friction coefficient reducing component” means that the magnetic recording medium and the head when recording or reproducing a magnetic signal are formed by forming appropriate protrusions on the surface of the magnetic layer. It shall mean the component which has the effect of reducing the friction coefficient produced when contacting. Examples of the friction coefficient reducing component include nonmagnetic inorganic particles and carbon black. In the present invention, carbon black is not included in the nonmagnetic inorganic particles functioning as a friction coefficient reducing component in the magnetic layer.

  Examples of inorganic substances constituting the nonmagnetic inorganic particles include metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. Specifically, α-alumina, β-alumina, γ-alumina, θ-alumina, silicon dioxide, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, goethite, corundum, nitridation with an α conversion rate of 90% or more Silicon, titanium carbide, titanium dioxide, tin oxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide, etc. are used alone or in combination. be able to. In view of availability of particles having excellent size distribution and dispersibility, inorganic oxides are preferable, and silica (silicon dioxide) is more preferable.

As the nonmagnetic inorganic particles, colloidal particles are preferably used from the viewpoint of dispersibility. As the colloidal particles, inorganic oxide colloidal particles are more preferable from the viewpoint of availability. Examples of the inorganic oxide colloidal particles include colloidal particles of the above inorganic oxides. Specific examples include SiO ₂ · Al ₂ O ₃ , SiO ₂ · B ₂ O ₃ , TiO ₂ · CeO ₂ , SnO _{2. · Sb 2 O 3, SiO 2} · Al 2 O 3 · TiO 2, may be mentioned composite inorganic oxide colloidal particles such as _{TiO 2 · CeO 2 · SiO 2} . Preferable examples include inorganic oxide colloidal particles such as SiO ₂ , Al ₂ O ₃ , TiO ₂ , ZrO ₂ , and Fe ₂ O _{3. From} the viewpoint of easy availability of monodispersed colloidal particles, silica Colloidal particles (colloidal silica) are particularly preferred. For details of the colloidal particles, for example, paragraph [0066] of JP2012-164410A can also be referred to.

  The average particle size of the nonmagnetic inorganic particles is preferably not less than the magnetic layer thickness and more preferably not less than 1.2 times from the viewpoint of forming appropriate protrusions that contribute to reducing the friction coefficient on the surface of the magnetic layer. preferable. Further, from the viewpoint of preventing a spacing loss due to excessive protrusion of the nonmagnetic inorganic particles, the magnetic layer thickness is preferably 2 times or less, more preferably 1.7 times or less. In order to obtain even more excellent electromagnetic conversion characteristics, the average particle size of the nonmagnetic inorganic particles is preferably in the range of 50 to 200 nm. The magnetic layer thickness is preferably optimized according to the saturation magnetization amount, head gap length, and recording signal band of the magnetic head to be used. From the viewpoint of improving electromagnetic conversion characteristics, the thickness of the magnetic layer is preferably 200 nm or less, more preferably 170 nm or less, still more preferably 80 nm or less, and a viewpoint of forming a uniform magnetic layer. Is preferably 10 nm or more, more preferably 30 nm or more, and even more preferably 50 nm or more. The average particle size of the nonmagnetic inorganic particles is a value measured by the method described in paragraph [0068] of JP2012-164410A.

  The content of the nonmagnetic inorganic particles in the magnetic layer is preferably set in a range in which both excellent electromagnetic conversion characteristics and a reduced friction coefficient can be achieved. Specifically, the content is 0 with respect to 100 parts by mass of the ε-iron oxide powder. It is preferable to set it as 5-20 mass parts, It is more preferable to set it as 1-5 mass parts, It is still more preferable to set it as 1-3 mass parts.

By the way, a magnetic layer containing an aromatic compound in which a hydroxyl group is directly substituted on an aromatic ring as a compound having a substituent selected from the group consisting of a hydroxyl group and a quaternary ammonium base is used in a coating type magnetic recording medium. It is preferable not to use carbon black, which is widely used as a magnetic layer component, in combination. This is because an aromatic compound in which a hydroxyl group is directly substituted with an aromatic ring easily binds to carbon black, so that carbon black and ε-iron oxide particles are associated through the aromatic compound to form coarse aggregates. It is thought that it is because it forms.
Therefore, a magnetic layer containing an aromatic compound in which a hydroxyl group is directly substituted with an aromatic ring contains nonmagnetic inorganic particles, preferably inorganic oxide colloidal particles, more preferably silica colloidal particles as the friction coefficient reducing component. It is desirable not to contain carbon black. Here, “not containing carbon black” means that it is not actively added as a component of the magnetic layer. For example, in the manufacturing process of a magnetic recording medium, as a component of another layer (for example, a nonmagnetic layer) It is allowed that the carbon black contained is unintentionally mixed in the magnetic layer.

Nonmagnetic Layer The coating type magnetic recording medium of the present invention can have a nonmagnetic layer containing a nonmagnetic powder and a binder between the nonmagnetic organic support and the magnetic layer. The nonmagnetic powder that can be used in the nonmagnetic layer may be an inorganic substance or an organic substance. Carbon black or the like can also be used. Examples of the inorganic substance include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. These nonmagnetic powders are available as commercial products, and can also be produced by a known method. Details thereof can be referred to paragraphs [0036] to [0039] of JP2010-24113A.

  As the binder, lubricant, dispersant, additive, solvent, dispersion method and the like of the nonmagnetic layer, those of the magnetic layer can be applied. In particular, known techniques relating to the magnetic layer can be applied to the amount of binder, type, additive, and amount of dispersant added, and type. Further, carbon black or organic powder can be added to the nonmagnetic layer. For example, paragraphs [0040] to [0042] of JP 2010-24113 A can be referred to.

Non-magnetic support Examples of the non-magnetic support include known ones such as biaxially stretched polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamideimide, and aromatic polyamide. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferable.
These supports may be subjected in advance to corona discharge, plasma treatment, easy adhesion treatment, heat treatment and the like. The surface roughness of the nonmagnetic support that can be used in the present invention is preferably a center average roughness Ra of 3 to 10 nm at a cutoff value of 0.25 mm.

Layer Configuration As for the thickness configuration of the magnetic recording medium of the present invention, the thickness of the nonmagnetic support is preferably 3 to 80 μm. The thickness of the magnetic layer is as described above. There may be at least one magnetic layer, and the magnetic layer may be separated into two or more layers having different magnetic characteristics, and a configuration related to a known multilayer magnetic layer can be applied.

  The thickness of the nonmagnetic layer is, for example, 0.1 to 3.0 μm, preferably 0.3 to 2.0 μm, and more preferably 0.5 to 1.5 μm. Note that the nonmagnetic layer of the magnetic recording medium of the present invention exhibits its effect if it is substantially nonmagnetic. For example, even if it contains a small amount of magnetic material as an impurity or intentionally, This shows the effect of the invention and can be regarded as substantially the same configuration as the magnetic recording medium in the present invention. Note that “substantially the same” means that the residual magnetic flux density of the nonmagnetic layer is 10 mT or less or the coercive force is 7.96 kA / m (100 Oe) or less, and preferably has no residual magnetic flux density and coercive force. Means.

Backcoat layer In the present invention, a backcoat layer can be provided on the surface of the nonmagnetic support opposite to the surface having the magnetic layer. The back coat layer preferably contains carbon black and inorganic powder. For the binder and various additives for forming the backcoat layer, the formulation of the magnetic layer and the nonmagnetic layer can be applied. The thickness of the back coat layer is preferably 0.9 μm or less, and more preferably 0.1 to 0.7 μm.

Production process The process for producing the coating liquid for forming each layer of the magnetic recording medium of the present invention such as a magnetic layer, a nonmagnetic layer, and a backcoat layer is usually at least a kneading process, a dispersion process, and before and after these processes. It consists of the mixing process provided as needed. Each process may be divided into two or more stages. Ε-iron oxide powder, non-magnetic powder, compound having a substituent selected from the group consisting of hydroxyl group and quaternary ammonium base (dispersant), binder, friction coefficient reducing component, carbon black, abrasive used in the present invention All raw materials such as antistatic agents, lubricants and solvents may be added at the beginning or during any step. In addition, individual raw materials may be added in two or more steps. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion. In order to achieve the object of the present invention, a conventional known manufacturing technique can be used as a partial process. In the kneading step, it is preferable to use a kneading force such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Further, glass beads or other beads can be used to disperse each layer forming coating solution. As such a dispersion medium, zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media, are suitable. The particle diameter and filling rate of these dispersion media are optimized. A well-known thing can be used for a disperser. For details of the method of manufacturing the magnetic recording medium, reference can be made, for example, to paragraphs [0051] to [0057] of JP 2010-24113 A. In addition, as described above, in the preparation process of ε-iron oxide powder, the dispersant can be used by adhering to ε-iron oxide powder by adding to the aqueous solution during or after washing. is there.

  The magnetic recording medium of the present invention described above can be suitably used as a magnetic recording medium for high-density recording because it can have a magnetic layer exhibiting high surface smoothness including ε-iron oxide powder having high thermal stability. is there. According to the present invention, the center line average roughness (Ra), which includes ε-iron oxide and is measured using an atomic force microscope (AFM), is 1.0 to 2.5 nm, and further 1.0. It is possible to provide a magnetic recording medium having a magnetic layer exhibiting surface smoothness of ˜2.0 nm.

  Hereinafter, the present invention will be described more specifically with reference to examples. It should be noted that the components, ratios, operations, order, and the like shown here can be changed without departing from the spirit of the present invention, and should not be limited to the following examples. “Parts” and “%” described below represent “parts by mass” and “% by mass”. The room temperature described below is 25 ° C. ± 1 ° C., and each operation was performed at room temperature unless otherwise specified.

Preparation Example 1: Synthesis of unsubstituted ε-iron oxide [Procedure 1: Preparation of micelle solution]
Two kinds of micelle solutions, micelle solution I and micelle solution II, were prepared by the following method.
(1) Preparation of micelle solution I After adding 107.9 g of pure water to 10.46 g of iron (III) nitrate nonahydrate and 123.7 g of cetyltrimethylammonium bromide, 439.8 g of n-octane, 1-butanol 101.2 g was added and stirred to dissolve.
(2) Preparation of micelle solution II To 123.7 g of cetyltrimethylammonium bromide, 178.5 g of 10% aqueous ammonia, 439.8 g of n-octane, and 101.2 g of 1-butanol were added and dissolved by stirring.

[Procedure 2: Precipitation of precursor particles]
The micelle solution II was added dropwise to the micelle solution I while stirring. After completion of the dropwise addition, the mixture was continuously stirred for 30 minutes.

[Procedure 3: Coating of precursor particles with sintering inhibitor]
In the mixed liquid obtained in the procedure 2, iron hydroxide Fe (OH) _{2 as} precursor particles is precipitated. While stirring the mixed solution, 48.9 g of tetraethoxysilane (TEOS) was added to the mixed solution. Stirring continued for about 1 day. Thereby, TEOS hydrolyzes and silica adheres to the surface of the precursor particles in the mixed solution.

[Procedure 4: Cleaning]
Place the solution obtained in step 3 into a separatory funnel, add 200 ml of a 1: 1 mixture of pure water and ethanol, let stand, wait for it to separate into a reddish brown part and other parts, except for the reddish brown part Abandoned. This operation was repeated three times, and then set in a centrifuge and centrifuged. The precipitate obtained by this treatment was collected. The collected precipitate was redispersed using a mixed solution of chloroform and ethanol, and centrifuged to collect the resulting precipitate.

[Procedure 5: Heat firing]
The precipitate obtained in procedure 4 was dried by air drying and then pulverized in a mortar. Thereafter, heat treatment was performed for 2 hours at an oven temperature of 1000 ° C. while sending air at 1 L / min in an image furnace manufactured by ULVAC-RIKO. As a result, ε-iron oxide particles to which silica as a sintering inhibitor is attached are obtained.

[Procedure 6: Removal of sintering inhibitor]
1 g of the ε-iron oxide particles adhering to silica obtained by the procedure 5 was put in 25 cc of 5N sodium hydroxide aqueous solution, treated for 4 hours while applying ultrasonic waves at a temperature of 70 ° C., and then stirred overnight. Thus, silica was removed from the surface of the ε-iron oxide particles.
Thereafter, washing with water and centrifugation were repeated, washing was performed until the pH of the supernatant was less than 8, and then dried by air drying to obtain ε-iron oxide particles. It was confirmed by powder X-ray diffraction analysis by X'Pert PRO (manufactured by PANalytical, radiation source CuKα ray, voltage 45 kV, current 40 mA) that the obtained particles were ε-iron oxide.

Preparation Example 2: Synthesis of Al-substituted ε-iron oxide Same as Preparation Example 1 except that the micelle solution I was prepared by the following method and the furnace temperature in Procedure 5 was 1025 ° C. By this method, an Al-substituted ε-iron oxide powder in which a part of Fe was replaced with Al was obtained.
<Preparation of micelle solution I>
After adding 207.9 g of pure water to 8.37 g of iron (III) nitrate nonahydrate, 1.94 g of aluminum nitrate and 123.7 g of cetyltrimethylammonium bromide, 439.8 g of n-octane and 101.1 of 1-butanol were added. 2 g was added and stirred to dissolve.

[Examples 1 to 4, Comparative Examples 1 and 2]
1-1. Magnetic layer coating liquid formulation ε-iron oxide powder listed in Table 1: 100 parts Polyurethane resin (functional group: —SO ₃ Na, functional group concentration 180 eq / t): 14 parts
Oleic acid: 1.5 parts 2,3-dihydroxynaphthalene: see Table 1 Alumina powder (average particle size: 120 nm): 50 parts Silica colloidal particles (colloidal silica; average particle size 100 nm): 2 parts Cyclohexanone: 110 parts Methyl ethyl ketone: 100 parts Toluene: 100 parts Butyl stearate: 2 parts Stearic acid: 1 part

1-2. Nonmagnetic layer coating liquid formulation Nonmagnetic inorganic powder (α-iron oxide): 85 parts Surface treatment agent: Al ₂ O ₃ , SiO ₂
Long axis diameter: 0.05μm
Tap density: 0.8
Needle ratio: 7
BET specific surface area: 52 m ² / g
pH 8, DBP oil absorption: 33g / 100g
Carbon black 20 parts DBP oil absorption: 120ml / 100g
pH: 8
BET specific surface area: 250 m ² / g
Volatile content: 1.5%
Polyurethane resin (functional group: —SO ₃ Na, functional group concentration: 180 eq / t): 15 parts Phenylphosphonic acid: 3 parts α-Al ₂ O ₃ (average particle size 0.2 μm): 10 parts Cyclohexanone: 140 parts Methyl ethyl ketone : 170 parts Butyl stearate: 2 parts Stearic acid: 1 part

1-3. Preparation of Magnetic Tape Each component was kneaded for 60 minutes with an open kneader for each of the above coating liquids, and then dispersed with a sand mill using zirconia beads (particle size 0.5 mm or 0.1 mm) for 720 to 1080 minutes. . 6 parts of a trifunctional low molecular weight polyisocyanate compound (Coronate 3041 manufactured by Nippon Polyurethane) was added to the obtained dispersion, and the mixture was further stirred and mixed for 20 minutes, followed by filtration using a filter having an average pore size of 1 μm, and coating with a magnetic layer. And a nonmagnetic layer coating solution were prepared.
The nonmagnetic layer coating solution was applied onto a 5 μm thick polyethylene naphthalate base so that the thickness after drying was 1.5 μm, and dried at 100 ° C. Immediately thereafter, the magnetic layer coating solution was wet-on-dried so that the thickness after drying was 0.08 μm, and dried at 100 ° C. At this time, vertical magnetic field orientation was performed with a 300 mT (3000 gauss) magnet while the magnetic layer was not dried. Furthermore, after performing a surface smoothing treatment at a speed of 100 m / min, a linear pressure of 300 kg / cm, and a temperature of 90 ° C. with a seven-stage calendar composed only of metal rolls, a heat curing treatment is performed at 70 ° C. for 24 hours. A magnetic tape was prepared by slitting to a width of 2 inches.

2. 2. Evaluation of magnetic tape 2-1. Coercivity A superconducting vibration magnetometer (VSM) manufactured by Tamagawa Seisakusho was used, and evaluation was performed under the condition of an applied magnetic field of 3184 kA / m (40 kOe).
2-2. Magnetic layer surface roughness Ra
Using an atomic force microscope (AFM: NANOSCOPE III manufactured by DIGITAL INSTRUMENT), an area of 40 μm × 40 μm was measured on the surface of the magnetic layer in a contact mode, and a center line average surface roughness (Ra) was measured.

  The results are shown in Table 1.

From the comparison between Examples 1 to 4 and Comparative Examples 1 and 2, by using the dispersant, ε-iron oxide powder could be highly dispersed, so that the magnetic layer surface Ra was low and excellent surface smoothness. It can be confirmed that the formation of a magnetic layer having the above has become possible.
Moreover, from the results shown in Table 1, it was possible to produce a magnetic tape exhibiting high coercive force by including ε-iron oxide powder, and by substituting a part of Fe in ε-iron oxide with Al. It can also be confirmed that the coercivity can be adjusted.

2. Evaluation of magnetic tape 2-3. Running durability (measurement of friction coefficient)
For Examples 1 to 4 and Comparative Examples 1 and 2 shown in Table 1, when the friction coefficient was measured by the following method, the friction coefficients (μ values) of Examples 1 and 2 and Comparative Examples 1 and 2 were:
Example 1: 0.24, Example 2: 0.22, Comparative Example 1: 0.19, Comparative Example 2: 0.20.
On the other hand, in Examples 3 and 4, the following cylindrical SUS rod stuck to the surface of the magnetic layer with a high friction coefficient, making reciprocal sliding difficult.
From the above results, it can be confirmed that in order to obtain a magnetic recording medium excellent in running durability in addition to the surface smoothness of the magnetic layer, the friction coefficient reducing component should be used as the magnetic layer component.
<Friction coefficient (μ value) measurement method>
Friction coefficient (μ) when the surface of the magnetic layer of the magnetic tape repeatedly slides 100 times with a load of 100 g against a cylindrical SUS rod having a center line average surface roughness Ra of 5 nm measured by AFM at a speed of 10 mm / sec. Value).

[Examples 5 and 6, Comparative Examples 3 and 4]
Magnetic tapes were prepared and evaluated in the same manner as in Examples 1 and 2 and Comparative Examples 1 and 2, except that 2 parts of carbon black (average particle size of 15 nm) was used in place of colloidal silica as a friction coefficient reducing component. . The results are shown in Table 2.

In Comparative Examples 3 and 4, since 2,3-dihydroxynaphthalene was not contained in the magnetic layer, the ε-iron oxide powder could not be sufficiently dispersed. The reason is also high.
On the other hand, the reason why the magnetic layer surface Ra was higher in Examples 5 and 6 than in the examples shown in Table 1 was that carbon black used as a friction reducing component was mixed with carbon black and ε via 2,3-dihydroxynaphthalene. -It can be considered that the iron oxide particles are associated.
On the other hand, as described above, Examples 3 and 4 that do not contain a friction coefficient reducing component showed results inferior in running durability.
From the above results, in order to improve the dispersibility of the ε-iron oxide powder, when using an aromatic compound in which a hydroxyl group is directly substituted with an aromatic ring, carbon black is not used as a magnetic layer component, and silica colloid is used. It can be confirmed that the use of a friction coefficient reducing component other than carbon black such as particles is desirable for obtaining a magnetic recording medium having good running durability in addition to the surface smoothness of the magnetic layer.

[Example 7]
As the ferromagnetic powder of the magnetic layer, 100 parts of the CTAB-coated Al-substituted ε-iron oxide powder prepared in Preparation Example 3 below (in terms of solid content of the wet cake prepared in Preparation Example 3) was used, and the magnetic layer component was 2 The magnetic tape was prepared and evaluated in the same manner as in Examples 1 and 2, except that, 3-dihydroxynaphthalene was not added. The evaluation results are shown in Table 3.

Preparation Example 3: CTAB-deposited Al-substituted ε-iron oxide synthesis procedure 6 was carried out in the same manner as in Preparation Example 2 except that the synthesis procedure 6 was changed as follows. A coated Al-coated ε-iron oxide was prepared.
[Procedure 6: Removal of sintering inhibitor, CTAB deposition treatment]
1 g of the ε-iron oxide particles adhering to silica obtained by the procedure 5 was put in 25 cc of 5N sodium hydroxide aqueous solution, treated for 4 hours while applying ultrasonic waves at a temperature of 70 ° C., and then stirred overnight. Thus, silica was removed from the surface of the ε-iron oxide particles.
Thereafter, washing with water and centrifugation were repeated, and washing was performed until the pH of the supernatant was less than 8. To the washed aqueous solution, 20 cc of a 1% by mass aqueous solution of CTAB per 1 g of particles contained in the aqueous solution (which was slightly suspended) was irradiated with ultrasonic waves, centrifuged, and the supernatant was discarded. Then, 20 cc of methyl ethyl ketone (MEK) was added, and after ultrasonic irradiation, centrifugation was repeated twice to obtain a CTAB-coated Al-substituted ε-iron oxide wet cake (without drying this wet cake, the magnetic Used to prepare the layer coating solution).

  From the comparison between Example 7 and Comparative Examples 1 and 2 shown in Table 1, it is possible to highly disperse the ε-iron oxide powder by using CTAB, which is an aliphatic compound having a quaternary ammonium base. Therefore, it can be confirmed that the magnetic layer having excellent surface smoothness with a low magnetic layer surface Ra can be formed.

  A magnetic tape was prepared and evaluated in the same manner as in Example 7 except that 2 parts of carbon black (average particle size of 15 nm) was used in place of colloidal silica as a friction coefficient reducing component. An increase in the magnetic layer surface Ra as seen in comparison with Examples 1 and 2 was not confirmed.

  The present invention is useful in the field of manufacturing magnetic recording media for high density recording.

Claims (4)

A magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a nonmagnetic support,
The ferromagnetic powder is ε-iron oxide powder,
The magnetic layer is a magnetic recording medium further comprising cetyltrimethylammonium bromide . The magnetic recording medium according to claim 1, wherein the magnetic layer further includes a friction coefficient reducing component. The magnetic recording medium according to claim 2 , wherein the friction coefficient reducing component is inorganic oxide colloidal particles . The magnetic recording medium according to claim 2 , wherein the friction coefficient reducing component is silica colloid particles .